So sorry you and your little guys are going through this. Do you have access to peppermint oil or another oil that repels insects like ants? You obviously can't put it in with the frogs for safety reasons, but if you put it around their enclosure and where the ants are getting in, hopefully that can help keep them away.

Another effective solution would be to bite the bullet and just fully adopt them by giving them an indoor enclosure. You seem very invested in their well-being and this would insure they will likely reach adulthood. Otherwise...this is kind of the circle of life in the wild. There's a reason critters like rabbits and frogs breed in high numbers; they're prey animals and only a few out of a large number make it to adulthood for many reasons.